1. Field of the Invention
This invention relates to methods and compositions for treating fibrosis. More particularly, the invention relates to the use of angiotensin inhibitors in combination with nitric oxide stimulators to inhibit the progression of fibrosis.
2. Description of the Prior Art
Fibrosis, the formation of excessive amounts of fibrotic or scar tissue, is a central issue in medicine. Scar tissue blocks arteries, immobilizes joints and damages internal organs, wreaking havoc on the body's ability to maintain vital functions. Every year, about 1.3 million people are hospitalized due to the damaging effects of fibrosis, yet doctors have few therapeutics to help them control this dangerous condition. As a result, they often see patients crippled, disfigured or killed by unwanted masses of uncontrollable scars.
Fibrosis can follow surgery in the form of adhesions, keloid tumors or hypertrophic (very severe) scarring. Fibrosis causes contractures and joint dislocation following severe burns, wounds or orthopaedic injuries; it can occur in any organ and accompanies many disease states, such as hepatitis (liver cirrhosis), hypertension (heart failure), tuberculosis (pulmonary fibrosis), scleroderma (fibrotic skin and internal organs), diabetes (nephropathy) and atherosclerosis (fibrotic blood vessels).
Ironically, the very process designed to repair the body can lead to dangerous complications. Like epoxy, scar tissue serves only a structural role. It fills in the gaps, but cannot contribute to the function of the organ in which it appears. For example, as fibrotic scar tissue replaces heart muscle damaged by hypertension, the heart becomes less elastic and thus less able to do its job. Similarly, pulmonary fibrosis causes the lungs to stiffen and decrease in size, a condition that can become life-threatening. Fibrotic growth can also proliferate and invade the healthy tissue that surrounds it even after the original injury heals. Too much scar tissue thus causes physiological roadblocks that disfigure, cripple or kill.
In most cases, fibrosis is a reactive process, and several different factors can apparently modulate the pathways leading to tissue fibrosis. Such factors include the early inflammatory responses, local increase in fibroblast cell populations, modulation of the synthetic function of fibroblasts, and altered regulation of the biosynthesis and degradation of collagen.
One treatment approach, therefore, has been to target the early inflammatory response. Treatment with topical corticosteroids has achieved limited success, if used early in fibrosis. However, steroid therapy has little or no effect once scar tissue has already formed. Furthermore, prolonged administration of hydrocortisone, in pulmonary fibrotic disease for example, may actually worsen the condition.
The second approach involves slowing the proliferation of those cells responsible for the increased collagen synthesis. Generally, this involves fibroblast cells, except in the vasculature where smooth muscle cells are responsible for collagen deposition. Compounds that have been used to inhibit fibroblast proliferation include benzoic hydrazide, as taught by U.S. Pat. No. 5,374,660. Benzoic hydrazide has been shown to suppress collagen synthesis and fibroblast proliferation, at least in tissue culture cells. U.S. Pat. No. 5,358,959 teaches the use of imidazole derivatives to inhibit the growth of fibroblasts by blocking the calcium-activated potassium channel. This particular agent also inhibits the proliferation of endothelial cells and vascular smooth muscle cells.
Likewise, a number of agents which affect smooth muscle cell proliferation have been tested. These compositions have included heparin, coumarin, aspirin, fish oils, calcium antagonists, steroids, prostacyclin, rapamycin, dipyridamole, ultraviolet irradiation, gamma (.gamma.)-interferon, serotonin inhibitors, methotrexate and mycophenolic acid, either alone or in various combinations.
A number of treatments have been devised that are based on the modulation of the synthetic function of fibroblast or smooth muscle cells. Like most cells, fibroblasts and smooth muscles cells are modulated by cytokines (factors secreted in response to infection that modify the function of target cells). Gamma interferon is a lymphokine (a cytokine that is produced by lymphocytes) known to inhibit fibroblast proliferation and collagen synthesis. Likewise, the monokine (a cytokine that is produced by macrophages) beta-interferon serves the same function. Thus, U.S. Pat. No. 5,312,621 teaches the use of these cytokines in the treatment of fibrosis. Similarly, certain cytokines have been tested for their effect on the proliferation and stimulation of collagen synthesis in smooth muscle cells. For example, U.S. Pat. No. 5,268,358 is directed to the use of peptides that block the binding of platelet-derived growth factors to their receptors. U.S. Pat. No. 5,304,541 is directed to chimeric transforming growth factor-beta (TGF-.beta.) peptides which block cell proliferation. U.S. Pat. No. 5,308,622 is directed to conjugates comprising fibroblastic growth factor (FGF) and cytotoxic agents. U.S. Pat. No. 5,326,559 is directed to interleukin-2 targeted molecules. Although promising, many of these agents and compositions have known and serious side effects and, consequently, limited effectiveness.
The final treatment strategy involves directly influencing the metabolism of collagen and the other components of fibrotic tissue. Thus, drugs that interfere with the biosynthesis, accumulation and catabolism of collagen have been used in the treatment of fibrosis. Many drugs are used to inhibit collagen synthesis, including derivatives of pyridone, alkadiene, benzoquinone, pyridine, oxalylamino acid and proline analogs. However, all of these drugs suffer from the drawback of also inhibiting the normal, and required, synthesis of collagen as well as the detrimental synthesis that occurs during fibrosis.
One of the most important pathologies for which fibrosis is a contributing factor is cardiovascular disease. Cardiovascular disease is the leading cause of death in the Western world. In the United States it accounted for 930,000 deaths in 1990. There are an estimated 1.5 million heart attacks per year in the U.S. that result in more than 500,000 deaths annually.
One consequence of heart disease is activation of the body's renin-angiotensin-aldosterone system (RAAS). The RAAS system maintains normal fluid volume in the body. The sympathetic nervous system provokes the release of the renin from the kidneys. The release of renin is stimulated by decreased extracellular fluid volume, low renal perfusion and decreased sodium content in the macula densa. Renin is a proteolytic enzyme that acts on angiotensinogen to produce the decapeptide angiotensin I. Angiotensin I is then converted to the octapeptide angiotensin II (AII) by the action of angiotensin-converting enzyme (ACE). AII is a potent pressor agent producing a rapid elevation in blood pressure. AII also plays a role in proliferation of smooth muscle cells.
A particular direct vasodilator is nitric oxide (NO). The use of NO donors is known for decreasing blood pressure in the treatment of angina, ischemic diseases, congestive heart failure, impotence in males, hypertension, arteriosclerosis, cerebral vasospasm, and coronary vasospasm (U.S. Pat. Nos. 4,954,526; 5,278,192).
NO is synthesized by NO synthase (NOS), of which there are several forms. When a dilator such as bradykinin acts at receptors on endothelial cells, the NOS is activated. The newly synthesized NO activates guanylate cyclase, thus producing cGMP. cGMP produces muscle relaxation by mechanisms that are not completely understood, but may involve cGMP-dependent protein kinase phosphorylation of myosin light chain which alters its contractility. NO is also implicated in platelet aggregation and adhesion and functions in various vascular reflexes.
NO therapy is generally achieved by administering drugs, such as nitroglycerine, that donate NO once inside the cell. The rest of the molecule (or NO degradation products) may be metabolically active, thus further complicating the problem of delineating the specific effect of NO on hypertension.
Nitroglycerine (NTG), an NO donor, has been known to be effective in the treatment of angina pectoris since 1879. However, patient tolerance to NTG is a problem, and increasing dosages are often required.
These and other disadvantages of the prior art are overcome by the present invention, and a method and compositions for the treatment of fibrosis is provided.